On the division of cortical cells into simple and complex types: a comparative viewpoint.

Hubel and Weisel introduced the concept of cells in cat primary visual cortex being partitioned into two categories: simple and complex. Subsequent authors have developed a quantitative measure to distinguish the two cell types based on the ratio between modulated responses at the stimulus frequency (F1) and unmodulated (F0) components of the spiking responses to drifting sinusoidal gratings. It has been shown that cells in anesthetized cat and monkey cortex have bimodal distributions of F1/F0 ratios. A clear local minimum or dip exists in the distribution at a ratio close to unity. Here we present a comparison of the distributions of the F1/F0 ratios between cells in the primary visual cortex of the eutherian cat and marsupial Tammar wallaby, Macropus eugenii. This is the first quantitative description of any marsupial cortex using the F1/F0 ratio and follows earlier papers showing that cells in wallaby cortex are tightly oriented and spatial frequency tuned. The results reveal a bimodal distribution in the wallaby F1/F0 ratios that is very similar to that found in the rat, cat, and monkey. Discussion focuses on the mechanisms that could lead to such similar cell distributions in animals with diverse behaviors and phylogenies.